1. Field of the Invention
The present invention relates to devices and methods for clamping the leads of a lead frame while the leads are being wire-bonded to bond pads on an integrated circuit die.
2. State of the Art
Integrated circuit (IC) dice or xe2x80x9cchipsxe2x80x9d are small, generally rectangular IC devices cut from a semiconductor wafer, such as a silicon wafer, on which multiple ICs have been fabricated. IC dice are typically packaged to protect them from corrosion by attaching them to lead frames using a solder or epoxy, electrically connecting them to leads in the lead frames, and then encapsulating the resulting die and lead frame assemblies in plastic die packages. The leads in the encapsulated lead frames protrude from the die packages and terminate in pins so the dice electrically connected to the leads can communicate with circuitry external to the die packages through the pins.
IC dice are typically electrically connected to the leads in their associated lead frames in a process known as xe2x80x9cwire-bonding.xe2x80x9d During wire-bonding of an IC die to a lead frame, a wire-bonding tool, such as an ultrasonic, thermosonic, or thermo-compression tool, bonds lengths of fine wire to bond pads on the die and to bonding sites on the leads in the lead frame in order to connect the bond pads and the leads. This process is well-known, and is described in detail in U.S. Pat. Nos. 4,030,657, 4,361,261, 4,527,730, 4,600,138, 4,603,803, 4,765,531, 4,778,097, 5,148,959, 5,217,154, 5,251,805, 5,307,978, 5,421,503, 5,425,491, 5,445,306, 5,465,899, 5,486,733, 5,513,792, and 5,516,023.
In order to ensure proper wire-bonding, lead frame leads are typically clamped during wire-bonding to stabilize them so a solid bond can be formed between the leads and the fine wire used to wire-bond them to an IC die. Without clamping, the leads can move during wire-bonding and cause unreliable bonds to be formed between the leads and the fine wire.
Conventionally, all the leads in a lead frame are clamped at once in a single plane during wire-bonding, as shown in U.S. Pat. Nos. 3,685,137, 5,035,034, and 5,193,733. By clamping all the leads of a lead frame at once, the conventional clamping method holds to a relative minimum the time it takes to wire bond an IC die to the lead frame.
While holding wire-bonding time to a minimum is advantageous, the conventional clamping method can also be problematic with certain lead frames. For example, some lead frames, such as the Leads-Over-Chip lead frame shown in U.S. Pat. No. 4,862,245, have leads which extend closer to the bond pads of IC dice attached to the lead frames than other leads such that fine wires bonded between the IC dice and these other leads must arch over the leads which are closer to the bond pads on the dice. Since fine wires must arch over the leads closer to the bond pads, the closer leads cannot be clamped at the same time all the other leads are clamped. Instead, the closer leads conventionally remain unclamped, or only partially clamped, during wire-bonding, resulting in potentially unreliable bonds to those leads.
Also, because the conventional clamping method clamps all the leads of a lead frame at once in a single plane, it cannot adequately clamp leads of a lead frame which must be clamped in different planes. For example, as shown in U.S. Pat. No. 4,943,843, some Tape-Under-Frame lead frames have leads which extend underneath an IC die and then project beyond the footprint of the die so they may be wire-bonded to bond pads on the die. These leads are typically left unclamped during wire-bonding, because they are not clampable in accordance with the conventional clamping method in the same plane as the remainder of the leads. As a result, these leads have potentially unreliable wire bonds. In a similar fashion, the conventional clamping method cannot adequately clamp multi-layer lead frames, such as that shown in U.S. Pat. No. 5,291,061.
Wire-bonding clamps, such as that shown in U.S. Pat. No. 4,821,945, have been developed to alleviate some of the problems of the conventional clamping method by clamping leads individually. However, these clamps can dramatically increase the time it takes to wire-bond an IC die, because the clamp must be sequentially indexed to each individual lead.
Therefore, there is a need in the art for a device and method for clamping lead frame leads during wire-bonding that can clamp leads in different planes without significantly increasing the time it takes to wire-bond an IC die.
An inventive wire-bonding apparatus includes a base for supporting a lead frame during wire-bonding. A clamp mechanism in the apparatus fixedly holds sets of leads of the lead frame against the base one set at a time, with each set including fewer than all the leads and at least one set including more than one of the leads. Also, a wire-bonding tool wire-bonds leads fixedly held against the base by the clamp mechanism to bond pads on an integrated circuit (IC) die. As a result, leads in different planes can be effectively clamped and wire-bonded in separate sets, yet the leads within each set are all clamped at the same time, so the wire-bonding time for an IC die is kept to a minimum.
Further embodiments include a clamping apparatus which includes the base and clamp mechanism described above and the clamp mechanism described above by itself.
In an additional embodiment, a bonding machine for wire bonding leads of lead frames in a lead frame strip to bond pads on IC dice attached to the lead frames includes a heat block with a contact area for heating the dice and the lead frames. An indexing mechanism indexes the lead frame strip through the bonding machine so each lead frame in the strip is sequentially indexed to the heat block""s contact area. A clamp mechanism fixedly holds sets of the leads of each lead frame indexed to the heat block""s contact area against the heat block one set at a time, and a wire-bonding tool wire-bonds the leads in each set held against the heat block to bond pads on the IC die attached to the lead frame of the held leads.
In another embodiment, a bonding machine similar to that described above simultaneously clamps and wire-bonds sequential lead frames in a lead frame strip at multiple contact areas on a heat block.
In a still further embodiment, a bonding machine for wire bonding leads of a Tape-Under-Frame (TUF) lead frame to bond pads on a front-side surface of an IC die attached to the TUF lead frame includes a heat block for supporting and heating the die and the TUF lead frame. A clamp mechanism fixedly holds a first set of leads of the TUF lead frame, which extends in a plane of a backside surface of the IC die to support the die, and a second set of leads of the TUF lead frame, which extends in the plane of the front-side surface of the IC die, against the heat block one set at a time. Also, a wire-bonding tool wire bonds the leads in each set of leads fixedly held against the heat block to bond pads on the IC die.
In still another embodiment, a bonding machine for wire bonding leads of a partial Leads-Over-Chip (LOC) lead frame to bond pads on an IC die includes a heat block for supporting and heating the die and the partial LOC lead frame. A clamp mechanism fixedly holds LOC leads of the partial LOC lead frame against the die, and other leads which terminate proximate the perimeter of the IC die against the heat block, at different times. Also, a wire bonding tool wire bonds the fixedly-held leads to bond pads on the IC die.
In yet another embodiment, a bonding machine for wire bonding bus bar and non-bus bar LOC leads of an LOC lead frame to bond pads on an IC die attached to and supported by the LOC leads includes a heat block for supporting and heating the die and the LOC lead frame. A clamp mechanism fixedly holds the bus bar and non-bus bar LOC leads against the die at different times, and a wire-bonding tool wire bonds the fixedly-held LOC leads to bond pads on the IC die.
In another additional embodiment, a bonding machine for wire bonding leads of a lead frame to bond pads on an IC die attached to the lead frame includes a heat block for supporting and heating the die and the lead frame. A clamp mechanism fixedly holds a first set of the leads of the lead frame, which extend toward the die and terminate proximate the die""s perimeter in a first plane, and a second set of the leads of the lead frame, which extend toward the die and terminate proximate the die""s perimeter in a second plane different than the first, against the heat block at different times. Also, a wire-bonding tool wire bonds the fixedly-held leads to bond pads on the IC die.
In a further embodiment, a method for clamping leads of a lead frame includes clamping a set of the leads at a time with each set including fewer than all the leads and at least one set including more than one of the leads.
In a still further embodiment, a method for wire bonding leads of lead frames in a lead frame strip to bond pads on IC dice attached to the lead frames includes: sequentially indexing each lead frame in the strip to a clamping position; clamping the leads of each lead frame indexed to the clamping position one set at a time with each set including fewer than all the leads of the lead frame and at least one set including more than one of the leads; and wire bonding each set of clamped leads to bond pads on the IC die attached to the lead frame of the clamped leads.